please, check the TagVisibility parameter of Goto flag - if it is set to "local" it is not visible to the MB driver. Also check the output of REXYGEN Compiler which will tell you details about wrong setup of flags.
I believe that's all you need. Now it's on you to build upon this concept.
Btw, what is your motivation for building your very own HMI? From your questions I guess that using the HMI Designer and building the HMI without any hand coding would be much easier and more straightforward for you.
if you mean 2 separate Raspberry Pi with Unipi 1.1 on top of each other, then yes, it is possible. It is a question of getting data from one device to the other. For this purpose you can use:
RDC function block (see example 0202-12)
REST API of REXYGEN (see example 0302-01)
Modbus TCP connection (see example 0402-01)